Stretch the stop, shrink the force

Science · Forces & Motion · ages 15-16

Name ______________________ Date ____________

Momentum change 1200 kg m/s, contact time 0.1 s. What is the average force in N?
Answer: ______________
What is the equation linking average force, time, and momentum change?
- Force = momentum change divided by time
- Force = momentum change times time
- Force = time divided by momentum change
A crumple zone changes the total momentum change in a crash.
Circle one: True False
How does an airbag protect the driver?
- It raises the impact speed
- It cuts the momentum change
- It stretches the stopping time
The same 1200 kg m/s change is stretched to 0.4 s. What is the average force in N?
Answer: ______________
Stopping time halves with the same momentum change. What happens to the force?
- It stays fixed
- It halves
- It doubles
A student writes: shorter time, same change, smaller force. What is wrong?
- Shorter time raises force, not lowers
- Same change is wrong
- Smaller force is wrong
A mat and a concrete floor stop the same fall. Why does the mat hurt less?
- It removes momentum
- Same change, longer time, smaller force
- It slows gravity

Answer key

For grown-ups. Fold this page away before handing over the rest.

12000 · 1200 divided by 0.1 = 12000 N.
Force = momentum change divided by time · Average force = momentum change divided by contact time.
False · The change stays fixed by the masses and speeds. Only the time stretches.
It stretches the stopping time · A longer stopping time with the same change means a smaller force.
3000 · 1200 divided by 0.4 = 3000 N: same change, quarter the force.
It doubles · Force divides by time, so half the time means double the force.
Shorter time raises force, not lowers · The chain runs: longer time, same change, smaller force.
Same change, longer time, smaller force · The fall fixes the change, the mat stretches the time, so the force falls.